1. Field of the Invention
The present invention relates to a method of detecting the angle of rotation of a DC motor, a method of detecting the load torque, and an apparatus of detecting the angle of rotation of a DC motor, an apparatus of detecting the load torque.
2. Description of the Related Art
In general, locking can occur in the operation of a motor when a trouble occurs in a motor itself or in a transmission mechanism such as a gear for transmitting power from a motor to a load. If such locking occurs in a motor used in a steering system of a car, a steering wheel will be locked, and it will become impossible to control the operation of the steering system.
One known technique to solve the above problem is to mechanically isolate the output shaft of the motor from the steering system by controlling the clutch when an abnormality is detected in the load torque of the motor estimated from the driving current and the angle of rotation or the angular acceleration velocity of the motor.
In the above technique, when neither the angle of rotation nor the angular velocity can be detected for some reason associated with the construction of the motor controller, the motor is driven by a constant current and the load torque is estimated by detecting armature current.
FIG. 10 illustrates the method of determining the load torque from the driving current and the angular velocity of a motor, described in "Motion Control" written by Dote and Harashima and published in March, 1993, by Corona Publishing Co. Ltd.
In FIG. 10, a current detector 2 detects a driving current supplied from a driving circuit 1 to a motor 3, and an angular velocity detector 5 constructed with for example a tachometer generator is connected to the motor 3 in such a manner as to detect the angular velocity of the motor 3 on the basis of the load 4 substantially represented by the torque constant of the motor, disturbing torque, the inertia of the motor, and viscous friction coefficient.
The driving current detected by the current detector 2 is supplied to the multiplier of a microcomputer 6 which includes, in addition to the multiplier 61, an adder 62, an operation circuit block 63, and other elements. The multiplier 61 calculates the product of the driving current and the torque constant of the motor. The result is supplied to one of two inputs of the adder 52. On the other hand, the angular velocity detected by the angular velocity detector 4 is supplied to the operation circuit block 63. In the operation circuit block 63, the angular velocity is multiplied by the predetermined inertia of the motor and the resultant product is differentiated. The differentiated value is then added with the product of the angular velocity and the predetermined viscous friction coefficient. The resultant sum is applied to the other input of the adder 62. The adder 62 subtracts the output of the operation circuit block 63 from the output of the multiplier 61. The result is passed through a low-pass filter 7 composed of for example a first-order delay circuit. Thus, the output 8 of the low-pass filter 7 presents an estimated value of the load torque.
The above conventional technique has several problems as described below.
That is, the above method of detecting the angle of rotation of a motor needs a dedicated angle detection device such as an encoder coupled to the motor.
Furthermore, in the conventional method of detecting the load torque of the motor, since the load torque is estimated while driving a motor by a constant current as described above, it is required to use a dedicated current detector and angular velocity detector such as a tachometer generator. If the system has no angular velocity detector such as a tachometer generator or a motor angle detector such as a potentiometer, it is difficult to estimate the load torque or the system will be very expensive even if possible.
For example, it may be possible to detect the angular velocity by detecting the voltage generated by a motor. In general, however, when the motor is being driven, the generated voltage is present in a form of a mixture with a voltage applied to the motor and it is difficult to detect the generated voltage separately from the applied voltage. Therefore, the difficulty in the detection of the load torque using such the technique results in complexity and high cost.
Gradual degradation in the performance of a motor or a gear system and thus a gradual increase in the load torque are more likely to occur than an abrupt failure due to for example incorporation of a foreign substance. Therefore, in the case where a motor is used in the steering system of a car, it is required to detect the load torque as accurately as possible to predict a failure in a car system before an actual occurrence. On the other hand, if a partial short circuit occurs in a coil of a motor, the torque generated by the motor current decreases for a certain time period during one rotation. This ununiformity in the generated torque will give an uncomfortable feeling to a user driving the car.
Accordingly, it is a general object of the present invention to solve the above problems. More specifically, it is an object of the present invention to provide a simple, inexpensive, and reliable method of detecting the angle of rotation and the load torque of a motor. It is another object of the present invention to provide a simple, inexpensive, and reliable apparatus of detecting the angle of rotation and the load torque of a motor.